1. Field of the Invention
The present invention relates to a pump which sucks and discharges fluid, and is preferably applied particularly to a pump for handling fluid with high kinematic viscosity, for example various types of pumps which are mounted on vehicles, particularly a pump used for a hydraulic pump in an anti-lock brake device or traction control device.
2. Description of the Related Art
As shown in FIG. 20, a piston pump 1405 having a structure provided with an intake check valve in which a ball needle 1401 is applied pressure with a spring 1403 is known as a conventional hydraulic pump in an anti-lock brake device for a vehicle. In this piston pump 1405, because the spring force of the spring 1403 acts upon the ball needle 1401, the intake resistance of the intake check valve is high, and the intake was difficult when the kinematic viscosity of the brake oil increased, for example when at low temperature.
In this manner, the conventional piston pump 1405 have the problem with a deterioration of volumetric efficiency in case that the kinematic viscosity of the fluid is high.
That is to say, as shown in FIG. 21, the possible intake volume QA of the conventional piston pump 1405 increases in the suction stroke from top dead center to bottom dead center of the piston, in the decreases and discharge stroke from bottom dead center to top dead center.
In case that the kinematic viscosity of the brake oil is low, because the fluid flows into the pump chamber 1409 while substantially following the increase in the possible intake volume QA in the above suction stroke, the intake quantity of the piston pump 1405 becomes sufficient and no discharge insufficiency occurs. However, if the kinematic viscosity of the brake oil becomes high, for example, low temperature or the like, the inflow speed of the brake oil (fluid) to the pump chamber 1409 declines, and so the inflow quantity QB does not follow the increase in the possible intake volume QA and an inflow delay occurs. Because the possible intake volume QA declines when the piston passes bottom dead center, as described above, the final inflow quantity QC of fluid per cycle of the piston to the pump chamber may fall greatly below the maximum value Q0 (corresponding to bottom dead center) of the possible intake volume QA.
FIG. 21 shows the inflow quantity QB of the fluid in case that the inflow speed is Q0/(t.sub.4 -t.sub.0) [cm.sup.3 /sec.], and the final inflow quantity QC [cm.sup.3 ] is only approximately 69% of the maximum value Q0 [cm.sup.3 ]. In other words, the volumetric efficiency of the pump has declined to approximately 69%.